A-LevelBiologyBiodiversity and conservation9700/42 · Oct/Nov 2020 · Q89 Marks

The soft rush plant, Juncus effusus, grows in many habitats in Northern Europe. An investigation was carried out to assess whether there was a relationship between the height of soft rush plants and the altitude at which they grow on exposed hillsides. The mean height of 10 soft rush plants was calculated at each of eight different altitudes. The results are shown in Table 8.1. [Table 8.1] The statistical test, Spearman's rank correlation (rs), was applied to find out if there was a relationship between the altitude and the mean height of the soft rush plants.

(a)Describe a method that could be used to estimate the population size of a mobile animal, such as a brown rat.[5]
(b)(i)The formula for calculating Spearman's rank correlation is: rs = 1 - (6 × ΣD²) / (n³ - n) • ΣD² is the sum of the differences between the ranks of the two samples • n is the number of samples. In this investigation the value of ΣD² is 164. Calculate the value of rs. Show your working and write your answer to two decimal places. rs =[2]
(b)(ii)Use your value for rs to evaluate the relationship between the altitude and the mean height of the soft rush plants.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[5] any five from:
[5] 1 mark-release-recapture ;
[5] 2 capture (sample of) rats and count them;
[5] 3 ref. to humane traps ;
[5] 4 mark so as to not adversely affect rats or describe method of marking such as, tagging / using dyes ;
[5] 5 return to where they were caught / AW ;
[5] 6 allow time to mix (with population) / AW ;
[5] 7 capture second sample;
[5] 8 count number caught and number marked ;
[5] 9 population size estimate (M₁) = (number in first sample (N₁)×number in second sample (N₂)) / number in second sample that are marked (M₂) ;
[5] 10 AVP; e.g. repeat
[2] rs = 1 - (6×164) / (8³-8) or 1 - 984 / 504 ;
[2] = 1 - 1.95 ;
[2] rs = -0.95 ;
[2] there is a strong correlation ;
[2] negative correlation / as the altitude increases the mean height of the soft rush decreases ;

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.